The present invention generally relates to methods and apparatus for the treatment of waste and more particularly to methods and apparatus for the treatment of waste using arc plasma-joule heated melter systems.
The disposal of municipal solid waste (MSW) and other waste has become a major issue over the past few decades due to space limitations for landfills and problems associated with siting new incinerators. In addition, increased environmental awareness has resulted in a major concern of many large metropolitan areas and to the country as a whole to ensure that the disposal of solid waste is properly handled. See e.g., USA EPA, The Solid Waste Dilemma: An Agenda for Action, EPA/530-SW-89-019, Washington, D.C. (1989).
Attempts have been made to reduce the volume and recover the energy content of MSW through incineration and cogeneration. The standard waste-to-energy incinerator will process the solid combustible fraction of the waste stream, produce steam to drive a steam turbine, and as a result of the combustion process produce a waste ash material. Typically, the ash is buried in a municipal landfill. Current trends and recent rulings, however, may require such material to be shipped to landfills permitted for hazardous waste. This will substantially increase ash disposal costs. Moreover, there is increased public concern about gaseous emissions from landfills and the possibility of contamination of groundwater. Another disadvantage associated with incinerator systems is the production of large quantities of gaseous emissions resulting in the need for costly air pollution control systems in an attempt to decrease emission levels to comply with requirements imposed by regulatory agencies.
In order to overcome the shortcomings associated with incinerator systems, attempts have been made in the prior art to utilize arc plasma torches to destroy toxic wastes. The use of arc plasma torches provides an advantage over traditional incinerator or combustion processes under certain operating conditions because the volume of gaseous products formed from the plasma arc torch may be significantly less than the volume produced during typical incineration or combustion, fewer toxic materials are in the gaseous products, and under some circumstances the waste material can be glassified. It should be understood that the phrases xe2x80x9cplasma torchxe2x80x9d or xe2x80x9cplasma arc torchxe2x80x9d are incorrectly used by some as interchangeable with xe2x80x9cplasma arcxe2x80x9d; plasma torch, or plasma arc torches, are not the same thing as a plasma arc and should not be confused with each other. Moreover, a graphite electrode is used in the present invention, as will be discussed in detail later, to avoid many of the problems and complexities of plasma torches or plasma arc torches.
For example, U.S. Pat. No. 5,280,757 to Carter et al. discloses the use of a plasma arc torch in a reactor vessel to gasify municipal solid waste. A product having a medium quality gas and a slag with a lower toxic element leachability is produced thereby. U.S. Pat. No. 4,644,877 to Barton et al. relates to pyrolytic destruction of polychlorinated biphenyls (PCBs) using a plasma arc torch. Waste materials are atomized and ionized by a plasma arc torch and are then cooled and recombined into gas and particulate matter in a reaction chamber. U.S. Pat. No. 4,431,612 to Bell et al. discusses a hollow graphite electrode transfer arc plasma furnace for treatment of hazardous wastes such as PCBs.
A process for remediation of lead-contaminated soil and waste battery material is disclosed in U.S. Pat. No. 5,284,503 to Bitler et al. A vitrified slag is formed from the soil. Combustible gas and volatized lead, which are formed from the waste battery casings, are preferably transferred to and used as a fuel for a conventional smelting furnace.
The systems proposed by Barton et al, Bell et al, Carter et al, and Bitler et al have significant disadvantages. For example, such disadvantages include insufficient heating, mixing and residence time to ensure high quality, nonleachable glass production for a wide range of waste feeds. Additionally, hearth size and feeder design are significantly limited since furnace walls must be relatively close to the arc plasma which is the only heat source. High thermal stress on the walls of the furnace often occurs as a result of the limitation on the hearth size.
Prior art arc plasma furnaces with metal electrodes further may be limited by short electrode lifetime when used at higher DC current. Therefore, to achieve higher power output, the arc potential must be raised by lengthening the arc. This results in radiative thermal losses to the furnace side walls and leads to metal electrode (torch) ineffectiveness. In addition, there are often difficulties associated with prior art transfer arc plasmas in start-up and restarting of such arc plasma systems when cold, nonelectrically conducting material is being processed.
Another disadvantage associated with prior art systems is the inefficient use of combustible gases produced during conversion of waste materials. For example, combustion of gases often does not result in a high conversion rate and is thus inefficient. In addition, the combustion of such gases frequently results in the emission of pollutants such as oxides of nitrogen (NOx) in amounts that render the process environmentally unattractive.
Thus, while such prior art attempts have been useful, there remains a need in the art for a robust, easy to operate waste conversion system which minimizes hazardous gaseous emissions and which maximizes conversion of a wide range of solid waste into useful energy and produces a product stream which is in a safe, stable form for commercial use or which does not require special hazardous waste considerations for disposal.
It would therefore be desirable to provide robust, user friendly and highly flexible methods and apparatus for processing and converting a wide range of waste materials into useful energy and stable products while minimizing hazardous gaseous emissions, thereby overcoming the shortcomings associated with the prior art.
U.S. Pat. Nos. 6,127,645 (Titus et al.), 6,160,238 (Titus et al.) and 6,215,678 (Titus et al.), all of which are incorporated by reference herein, are directed to overcoming these problems by utilizing AC powered joule heating and DC arc electrode melting in arc plasma-joule heated melter systems. However, there still remains a need to accomplish the operation of the AC powered joule heating and DC arc electrode melting in a more efficient way that uses less components.
An apparatus for waste conversion (e.g., an arc furnace) wherein the apparatus comprises: a tank for receiving the waste; a single electrode for supporting a DC arc to melt the waste in the tank and wherein the single electrode is coupled to a source of DC power; is a plurality of AC joule heating electrodes, coupled to a source of AC power, submerged in the melted waste, that maintains the melted waste in a molten state while simultaneously acting as DC counter electrodes and wherein the plurality of AC joule heating electrodes are coupled to the DC power source.
A method for converting waste wherein the method comprises the steps of: (a) disposing the waste in a tank for treatment; (b) applying a DC arc to melt the waste through a single electrode; (c) applying AC joule heating to maintain the waste in a molten condition through a plurality of AC joule heating electrodes submerged in the waste; and (d) utilizing the plurality of AC joule heating electrodes as DC counter electrodes of the DC arc.